1. Field of the Invention
The present invention relates to an image processing device and an image processing method suitable for a color copying machine and a color printer.
2. Description of the Related Art
In a color copying machine and a color printer, generally, each color is represented according to the amount of the output color material of each primary color composed of black (K), yellow (Y), magenta (M) and cyan (C). If a black character and others are printed on a color graphic form and a color picture, a part overlapped with a black character of a background is first blanked. As for image data having a black character A on a photograph image including YMC, a background area is equivalent to an area except the character A of the original photograph image and a black area is an area equivalent to a part of the character A. A character, a graphic form, a photograph and others are generically called an object.
In electrophotography, a process in which the color material of KYMC is adhered to paper is independently executed every color. Therefore, the relative position of an image in each color may be displaced by a few pixels due to mechanical positional precision, the variation of the speed of paper feeding and the displacement of a carriage belt. In the above example, a white contour not existing originally is formed in a direction of the contour of the black character A. In this specification, such a problem is called misregistration.
Also, in electrophotography, an electrostatic latent image is formed on a drum provided every color of KYMC and color material is absorbed. When an electrostatic latent image in Y, M, C of the above photograph image is formed, an electrostatic latent image electric charge in only a part of the character A of which is zero is formed. In this case, a phenomenon that a photograph image pales in a larger range than a range of misregistration occurs because slight electric charge moves from the periphery to a part in which electric charge is zero and because of the others. In this specification, such a problem is called a phenomenon that an edge pales.
FIGS. 7 and 16 respectively show an example in case the above problem occurs. In the example shown in FIG. 7, only the phenomenon that an edge pales occurs in a little large range and no misregistration occurs. Further, in FIG. 16, misregistration occurs in a little small range around a black image in the center in addition to the phenomenon that an edge pales. In electrophotography, when the total amount in case a plurality of color materials are overlapped exceeds allowable amount, a failure of fixing occurs, color material may peel from paper and a printer may be damaged. Such a problem is called an over state in color material total amount.
In Japanese Published Unexamined Patent Application No. Hei 10-52947, background color under a black object is checked every pixel, according to the result, the composition and amount of color material for outputting the black object are determined and hereby, technique for preventing the phenomenon that an edge pales is disclosed. That is, the black object is not output depending upon only black color material but color material in Y, M, C is suitably added according to the background color.
Also, in Japanese Published Unexamined Patent Applications No. Hei 6-339037 and No. Hei 7-212608, technique for preventing a void caused by misregistration by trapping the contour of an object (slightly overlapping the color material of both objects) is disclosed. That is, a series of processing that (1) the contour of the object is detected, (2) referring to an adjacent pixel of each contour, it is determined whether trapping is to be performed or not, (3) in consideration of a value of each adjacent pixel, a value of a target pixel is calculated is executed.
However, in technique disclosed in the Japanese Published Unexamined Patent Application No. Hei 10-52947, as background color is checked in units of pixel and according to the result, the composition and amount of color material are required to be determined, there is a problem that much time is required. As the composition and amount of color material for printing a black object differ depending upon a place as shown in FIG. 8 even if the color of the black object in original image data is fixed, there is a problem that a single object looks like a set of a plurality of black objects. Further, as the bit map information of a plurality of color materials is required to be all stored, enormous storage capacity is required.
Also, according to the technique disclosed in the Japanese Published Unexamined Patent Applications No. Hei 6-339037 and No. Hei 7-212608, misregistration which occurs in a relatively small range can be prevented by trapping, however, it is difficult to prevent a phenomenon that an edge pales which occurs in a larger range.